You always link files in html with a path relative to where the files is called from
if you in your public_html have an index.file that needs to include a css file named mystyle.css
you'd call it using this the name only if the css file also is in the public_html folder, no path is needed since it is in the same folder: mystyle.css
if the css file is in a folder called css, you'd include it like this: css/mystyle.css
cPanel doesn't have a file linking system, html rules for linking is the only you need to worry about